1. Field of the Invention
The invention relates to a device for detecting a gas or gas mixture with a first and a second gas sensor, wherein the first gas sensor is a MOSFET comprising a first source, a first drain, a first channel zone disposed between the latter elements, and a first gas sensitive layer capacitively coupled to the first channel zone, which layer contains palladium and reacts to a change in the concentration of the gas to be detected with a change in its work function, wherein the second gas sensor has in a semiconductor substrate a second source, a second drain, and a second channel zone between the latter elements, which channel zone is capacitively coupled via an air gap to a suspended gate, which suspended gate has a second gas sensitive layer that reacts to a change in the concentration of the gas to be detected with a change in its work function, wherein the second gas sensitive layer is disposed on a support layer and faces the air gap. The invention further relates to a method for manufacturing a device for detecting a gas or gas mixture.
2. Description of the Background Art
Such a device is disclosed in EP 1 620 717 B1. It has two different kinds of gas sensors. A first gas sensor is configured as a suspended gate field effect transistor (SGFET), and a second gas sensor as a so-called Lundström-FET.
The Lundström-FET has a first transistor tub of a first charge carrier type, which is integrated in the semiconductor substrate of a semiconductor chip. In the first transistor tub are embedded a first source and a first drain, which are doped with charge carriers of a second charge carrier type. Between the first source and the first drain is formed a first channel zone, on which a dielectric layer is arranged. The dielectric layer is coated with a first gas sensitive layer that contains palladium and reacts to a change in the concentration of the gas to be detected with a change in its work function. This change affects a first current flowing between the first source and the first drain via the first channel zone.
The SGFET has a second transistor tub of the first charge carrier type, which is monolithically integrated in the semiconductor substrate next to and laterally spaced apart from the first transistor tub. In the second transistor tub are embedded a second source and a second drain, which are doped with charge carriers of the second charge carrier type. Between the second source and the second drain is formed a second channel zone, which is capacitively coupled via an air gap to a suspended gate. The suspended gate is arranged on a support layer bridging the second channel zone and it comprises a second gas sensitive layer facing the second channel zone. The second gas sensitive layer contains palladium and/or platinum and reacts to a change in the concentration of the gas to be detected with a change in its work function. This change affects a second current flowing between the second source and the second drain via the second channel zone.
The SGFET and the Lundström-FET are connected to an evaluator integrated in the semiconductor substrate, which analyzes the measurement signals of the two kinds of gas sensors in order to detect the gas or gas mixture. The device has proven to be advantageous in the industry, mainly because the Lundström-FET and the SGFET have different gas detection characteristics, and because the gas or gas mixture can be detected with considerably greater precision by analyzing the measurement signals of the two kinds of gas sensors than with a device comprising only one kind of gas sensor.
However, a disadvantage resides in the device in that an irreversible alteration of the SGFET can occur under certain operating conditions, particularly when measuring very high gas concentrations, at high temperatures, and at high air humidity. This alteration can reduce the measurement accuracy of the device. A further disadvantage resides in the fact that the device has a relatively large chip surface compared to a device comprising only one gas sensor. Manufacturing the device is hence associated with corresponding [higher] costs.